Electrical insulators, materials and equipment

ABSTRACT

An elongate high voltage insulator ( 2 ) is formed of a rod or tube ( 4 ) of insulating material, with a pair of electrodes ( 6 ) spaced apart longitudinally thereof. At least part, and preferably the whole of the outer surface of the insulating material ( 4 ) is covered by a layer of material ( 8 ) comprising a particulate filler of varistor powder in a matrix having a switching electrical stress-controlling characteristic that is in electrical contact with each of the electrodes ( 6 ). The insulator core ( 4 ) may be made of porcelain, and the stress-controlling material ( 8 ) may comprise zinc oxide.

[0001] This invention relates to electrical insulators, materials, andequipment, for example an elongate high voltage insulator.

[0002] An insulator typically comprises an insulating core that extendsbetween two electrodes which, in operation, are maintained atsignificantly different electrical potentials, one of which may beearth. The insulating core may comprise a tube or a rod, which may bemade of a ceramic material or of glass fibre reinforced plasticsmaterial, for example. Typically in an electrical distribution system,one end of the insulator is maintained at earth potential, and the otherend is at the potential of the system, which may be 10 kV or above, forexample the 375 kV electricity distribution system of the UK. At highvoltages, the insulator serves to isolate the system from earth, and thehigher the operating voltage of the system, the longer the insulator hasto be in order to maintain the isolation. The electrical stress betweenthe insulator electrodes results in leakage current flowing over thesurface of the insulating material from high voltage to ground, and thusleads to a constant loss of power from the operating system.

[0003] It is an object of the present invention to provide an improvedinsulator.

[0004] In accordance with one aspect of the present invention, there isprovided a high voltage free-standing insulator comprising an elongatetube or rod of electrically insulating material having a pair ofelectrodes spaced apart longitudinally thereof, and a layer of materialcomprising a particulate filler of varistor powder in a matrix having aswitching electrical stress-controlling characteristic, wherein thestress-controlling material extends over part or substantially all ofthe outer surface of the insulating material and in electrical contactwith each of the electrodes.

[0005] By the term “free standing”, it is meant that the insulator mayform an insulator per se, that is to say without there being anelectrical conductor extending therethorough, or it may be disposedaround, that is to say not formed in situ onto, supporting electricalequipment that may itself contain an electrical conductor.

[0006] Advantageously, the varistor material is inorganic, for example aceramic or a metal oxide, and preferably comprises zinc oxide.

[0007] Although the stress-controlling material may lie directly incontact with the insulating material, it is also envisaged that it maybe spaced therefrom, for example by another layer of material. Theother, intermediate, layer of material may be a stress-controllingmaterial having a different voltage/current characteristic from the zincoxide varistor material, for example a linear characteristic (c=1, seebelow).

[0008] It is thus seen that in addition to the conventional electricallyinsulating tube or rod, the insulator of the present invention isprovided with an outer layer of stress-controlling material, preferablyin the form of particulate zinc oxide varistor powder in a matrix, thismaterial having a switching electrical stress-controllingcharacteristic. This material distributes the electrical stress alongthe outer surface of the insulator when operating at high voltage. Uponapplication of an excessively high voltage to one of the electrodes, forexample arising from a lightning strike, the material substantiallyinstantaneously switches to a conductive mode, whereby the electricalpower is safely dissipated to earth. The material then amicrometresostimmediately reverts to its insulating mode.

[0009] Such a non-linear material obeys a generalised form of Ohms Law:1=kV^(c), where c is a constant greater than 1, whose value depends onthe material under consideration.

[0010] Such a stress controlling characteristic is not only non-linearin respect of the variation of its a.c. electrical impedance, but alsoexhibits a switching behaviour, in that the graph of voltage applied tothe material versus current flowing therealong shows an abrupttransition, whereby below a predetermined electrical stress, dependenton the particular material, the stress-controlling material exhibitsinsulating behaviour substantially preventing the flow of any current,but when that electrical stress is exceeded, the impedance of thematerial drops substantially to zero in a very short time so that thetriggering high voltage on the one terminal can be conducted to theother terminal, usually at earth potential.

[0011] The insulator of the present invention is particularly suitablefor forming an insulator per se, whether it be a tension, suspension,cantilever, compression or torsional electrical insulator. However, theinsulator, with the electrically insulating material in the form of atube, is also suitable for being disposed around electrical equipment,such as the termination of a high voltage cable, around a bushing, aswitch, or a disconnector, for example. Such electrical equipment may besusceptible to flashover as a result of contamination on the outersurface, especially in combination with moisture which can lead to theformation of dry bands with consequential flashover, tracking anderosion, which can in extreme cases destroy the insulating material andbring about failure of the insulating function. Sparking also produceselectromagnetic interference. Also, flashover can result from thecombination of high field stress along the outer insulating surface of acable termination arising from electrically stresses within thetermination in combination with the voltage stress across dry bands.Conventionally, such flashovers are minimised by increasing the lengthof the insulator, and/or the thickness of the insulating material, whichhas the undesirable effect of increasing the overall physical size ofthe arrangement. In accordance with the present invention, however, thestress-control material applied to the outside of the insulator limitsthe electrical field strength on that insulating surface, which surfacemay otherwise be the transition between insulating material and air.

[0012] In the application to a high voltage cable termination, theinsulator may be disposed around the cut back of the conductive screenof the cable, being a high stress region. The application of theswitching varistor material allows a smaller diameter construction to beachieved, whilst maintaining the desired electric strength axially ofthe insulator.

[0013] The varistor, electrical stress grading material may be disposedover the entire length of the underlying insulating material, oralternatively only partially thereover. In the latter case, the stresscontrol material may be located in the regions of relatively highelectrical field strength near the electrodes and extending along theinsulation away therefrom.

[0014] Furthermore, a capacitive stress grading effect may be achievedby alternating bands of the stress control material with exposedunderlying bands of the insulating material.

[0015] An insulator in accordance with the present invention would beexpected to be subject to less electrical activity, corona discharging,arcing, and material deterioration, and to exhibit better flashoverresistance than a conventional insulator, particularly in ambientconditions of high humidity and/or contamination.

[0016] The stress-controlling layer used in the invention may comprisethe outermost layer of the insulator. Alternatively, thestress-controlling material may itself be enclosed within an outer layerthat provides electrical and/or environmental protection for theinsulator.

[0017] Provided that the substrate, insulating, material is ofsufficiently low thermal capacity and of sufficiently high thermalconductivity, it will conduct heat away relatively quickly from thevaristor material, so that an outer protective covering may not berequired. A ceramic, for example porcelain, substrate would be suitablein this respect. However, if the underlying insulating material were,for example, a silicone polymeric material, then in adverseenvironmental conditions, for example wet conditions, the amount ofleakage current may be high enough to degrade the varistor layer,requiring a protective external covering to be applied to the insulator.

[0018] The outermost component of the insulator is preferably providedwith one or more sheds, that is to say substantially disc-likeconfigurations that direct moisture and water and other contaminants offthe surface of the insulator so as to interrupt a continuous flowthereof from one electrode to the other, thus avoiding short-circuiting.

[0019] Preferably, the particles of the filler of the layer of stresscontrolling material are calcined at a temperature between 800° C. and1400° C., and subsequently broken up such that substantially all of theparticles retain their original, preferably substantially sphericalshape.

[0020] The calcination process is believed to result in the individualparticles effectively exhibiting a “varistor effect”. That is to say theparticulate material is not only nonlinear in respect of the variationof its a.c. electrical impedance characteristic (the relationshipbetween the a.c. voltage applied to the material and the resultantcurrent flowing therethrough), but it also exhibits a switchingbehaviour, in that the graph of voltage versus current shows an abrupttransition, which is quantified by the statement that the specificimpedance of the material decreased by at least fact of 10 when theelectric field is increased by less than 5 kV/cm (at some region withinan electric field range of 5 kV/cm to 50 kV/cm, and preferably between10 kV/cm and 25 kV/cm, —being a typical operating range of the materialwhen used in the termination of an electric power cable). preferably,the transition is such that the specified decrease takes place when theelectric field is increased by less than 2 kV/cm within the rangebetween 10 and 20 kV/cm. The non-linearity occurs in both the impedanceof the material and also in its volume resistivity. The non-linearity ofthe filler particles may be different on each side of the switchingpoint. It is also important that at the switching point the materialsimply significantly changes its non-linearity, and does not lead toelectrical breakdown or flashover as the electrical stress is increased.The smaller the particle size for any given composition, the less is thelikelihood of breakdown occurring beyond the switching point.

[0021] Preferably at least 65% of the weight of the filler compriseszinc oxide.

[0022] Preferably more than 50% by weight of the filler particles have amaximum dimension of between 5 and 100 micrometers, such that thematerial exhibits non-linear electrical behaviour whereby its specificimpedance decreased by at least a factor of 10 when the electric fieldis increased by less than 5 kV/cm at a region within an electrical fieldrange of 5 kV/cm to 50 kV/cm.

[0023] Preferably the filler comprises between 5% and 60% of the volumeof the stress-controlling material layer, advantageously between 10% and40%, and most preferably between 30% and 33% of the volume.

[0024] In practice the particulate filler will comprise at least 65%,and preferably 70 to 75% by weight of zinc oxide. The remainingmaterial, dopants, may comprise some or all of the following forexample, as would be known to those skilled in the art of doped zincoxide varistor materials: Bi₂O₃, Cr₂O₃, Sb₂O₃, Co₂O₃, MnO₃, Al₂O₃, CoO,Co₃O₄, MnO, MnO₂, SiO₂, and trace amounts of lead, iron, boron, andaluminium.

[0025] The polymeric matrix may comprise elastomeric materials, forexample silicone or EPDM; thermoplastic polymers, for examplepolyethylene or polypropylene; adhesives for example those based onethylene-vinyl-acetate; thermoplastic elastomers; thixotropic paints;gels, thermosetting materials, for example epoxy or polyurethane resins;or a combination of such materials, including co-polymers, for example acombination of polyisobutylene and amorphous polypropylene.

[0026] The stress-controlling material may be provided in the form of aglaze or paint, which may be applied, for example, to a ceramicinsulator or other insulating substrate. Such stress-controlling glazeor paint, and electrical articles or equipment of all kinds(free-standing or not) to which such glaze or paint has been applied,are another aspect of the present invention.

[0027] According to a further aspect of the present invention, theparticulate material hereindisclosed, preferably zinc oxide, is mixed inits fired, or preferably unfired, state into a slurry, which is thenfired to form a glaze.

[0028] The slurry may, for example, comprise clay that upon firingproduces porcelain or other ceramic. Alternatively, the matrix intowhich the particles are deposited may be inorganic, for example being apolymer, an adhesive, a mastic or a gel.

[0029] It will be appreciated that, in these forms of the invention, itmay be the step of firing the slurry, glaze, or paint that produces thevaristor switching characteristic required of the stress-controllingmaterial, if that characteristic has not previously been imposed, orsufficiently imposed, on the particulate material.

[0030] The total composition of the stress-controlling material may alsocomprise other well-known additives for those materials, for example toimprove their processibility and/or suitability for particularapplications. In the latter respect, for example, materials for use aspower cable accessories may need to withstand outdoor environmentalconditions. Suitable additives may thus include processing agents,stabilizers, antioxidants and platicizers, for example oil.

[0031] The presence of the varistor material on the outer surface of theinsulating material in the insulator of the present invention tends toresult in leakage current flowing through the bulk of the materialrather than along the surface when a dry band is formed, thus avoidingthe problem of tracking. Furthermore, such stress grading material alsoallows the insulator to be made of lesser wall thickness and smallerdiameter for good electrical performance in comparison with conventionalinsulators. Thus, with an insulator of the present invention, atcomparatively low voltages, the leakage current will flow relativelyharmlessly along its outer surface due to the comparatively lowimpedance of the varistor material. Should the voltage increase above acertain value, the varistor material will then switch over to its highimpedance state and the leakage current will then pass through the bodyof the material without the formation of damaging carbonaceous tracks onits outer surface.

[0032] The stress-controlling material may be applied to the insulatingmaterial by extrusion, by moulding, or by being in the form of aseparate component. In the last-mentioned construction of the insulator,the stress-controlling material is preferably in the form of a tube, andmay advantageously, when the matrix comprises polymer, be recoverable,preferably heat-recoverable, into position. When the outer surface ofthe insulator is of shedded configuration, the sheds may be integrallyformed, or they may be applied separately.

[0033] International patent application publication number WO 97/26693discloses a composition for use as an electrical stress-controllinglayer, and that composition is suitable for the stress-controlling layerof the insulator of the present invention. The entire contents of thispublished patent application are included herein by this reference.

[0034] Two embodiments of insulator, each in accordance with the presentinvention, will now be described, by way of example, with reference tothe accompanying drawings, in which:

[0035]FIG. 1 shows a first embodiment in vertical section, in which astress-controlling layer of a hollow tubular insulator is enclosedwithin an outer protection layer;

[0036]FIG. 2 shows a second embodiment in which the stress-controllingmaterial is formed integrally with the outer protection layer of a solidcore insulator;

[0037]FIG. 3 is a graph of a typical particle size distribution of thecalcined doped zinc oxide filler; and

[0038]FIG. 4 is a graph of the impedance of the filler powder forvarious particle sizes.

[0039] Referring to FIG. 1, an insulator 2 comprises a cylindricaltubular core 4 of ceramic material, having a brass electrode 6 mountedon each end thereof. A layer of doped zinc oxide varistor material 8 ismoulded on to the entire outer surface of the insulating core 4 betweenthe electrodes 6. An optional outer protection layer 10 is applied tocover the entire outer surface of the stress-controlling layer 8. Theprotection layer 10 is provided with a pluraity of generally circularsheds 12 that project radially of the insulator 2. Core 4 mayalternatively be a solid body.

[0040] Referring to FIG. 2, the insulator 22 comprises an innercylindrical core 24 of fibre-reinforced epoxy resin extending between apair of terminal electrodes 26. In this embodiment, however, a single,shedded outer component 28 is moulded onto the core 24. The component 28is formed of a material that performs the function of controlling thestress on the outer surface of the insulator 24 as well as providingouter environmental protection therefor. The solid core 24 mayalternatively be a hollow tubular construction.

[0041] The doped zinc oxide stress-control material that forms the layer8 in the first embodiment (FIG. 1), and that is included in layer 28 ofthe second embodiment (FIG. 2) is a matrix of silicone elastomer and aparticulate filler of doped zinc oxide.

[0042] The doped zinc oxide comprises approximately 70 to 75% by weightof zinc oxide and approximately 10% of Bi₂O₃+Cr₂O₃+Sb₂O₃+Co₂O₃+MnO₃.

[0043] The powder was calcined in a kiln at a temperature of about 1100°C., before being mixed with pellets of the polymer matrix and fed intoan extruder to produce the final required form. The calcined fillercomprised about 30% of the volume of the total composition comprisingthe filler and the polymeric matrix.

[0044] A typical particle size distribution of relative numbers ofcalcined doped zinc oxide particles of a suitable powder, after havingbeen passed through a 125 micrometer sieve, is shown in FIG. 3, fromwhich it can be seen that there is a sharp peak at a particle size ofabout 40 micrometers, with the large majority of particles being between20 and 6 micrometers.

[0045] The switching behaviour of the calcined doped zinc oxideparticles, showing the abrupt change in non-linear specific impedance asa function of the electric field strength (at 50 Hz), is shown in FIG. 4for three ranges of particle size. Curve I relates to a particle size ofless than 25 micrometers, Curve II to a particle size of 25 micrometersto 32 micrometers and Curve III to a particle size of 75 micrometers to125 micrometers. It is seen that the switching point occurs at higherelectric field strength as the particle size is reduced.

[0046] It is envisaged that the inner insulating component correspondingto either core 4, 24 could be tubular, such that the insulator 2, 22could be mounted on, for example, the termination of a high voltagecable so as to provide protection against flashover along the outersurface thereof. In this embodiment it is also envisaged that thetermination of the cable itself would be stress-controlled, particularlyat the cut-back of the cable screen, as is done conventionally.

1. A free-standing high voltage insulator comprising an elongate tube orrod of electrically insulating material having a pair of electrodesspaced apart longitudinally thereof, and a layer of material comprisinga particulate filler of varistor powder in a matrix having a switchingelectrical stress-controlling characteristic, wherein thestress-controlling material extends over part or substantially all ofthe outer surface of the insulating material and at least some of thestress-controlling material is in electrical contact with each of theelectrodes.
 2. An insulator according to claim 1, wherein thestress-controlling material is present in two separate regions near andin electrical contact with the respective electrodes.
 3. An insulatoraccording to claim 1 or 2, wherein the stress-controlling materialcomprises inorganic material, preferably zinc oxide.
 4. An insulatoraccording to anyone of the preceding claims, wherein the layer ofstress-controlling material is enclosed within an outer layer thatprovides electrical and/or environmental protection therefor.
 5. Aninsulator according to any one of the preceding claims wherein the layerof stress-controlling material or the outer protection layer has ashedded outer configuration.
 6. An insulator according to any one of thepreceding claims, wherein (i) the particles of the filler of the layerof stress controlling material are calcined at a temperature between800° C. and 1400° C., and subsequently broken up such that substantiallyall of the particles retain their original shape, (ii) at least 65% ofthe weight of the filler comprises zinc oxide, (iii) more than 50% byweight of the filler particles have a maximum dimension of between 5 and100 micrometers, such that the material exhibits non-linear electricalbehaviour whereby its specific impedance decreases by at least a factorof 10 when the electric field is increased by less than 5 kV/cm at aregion within an electrical field range of 5 kV/cm to 50 kV/cm, and (iv)the filler comprises between 5% and 60% of the volume of thestress-controlling material layer.
 7. An insulator according to claim 6,wherein all the particles of the filler have a maximum dimension of lessthan 125 micrometers, preferably less than 100 micrometers.
 8. Aninsulator according to claim 6, or claim 7, wherein not more than 15% byweight of the filler particles have a maximum dimension less than 15micrometers.
 9. An insulator according to any one of claims 6 to 8,wherein the filler particles are calcined at a temperature between 950°C. and 1250° C., preferably at about 1100° C.
 10. An insulator accordingto any one of claims 6 to 9, wherein at least 70% of the weight of thefiller comprises zinc oxide.
 11. An insulator according to any one ofclaims 6 to 10, wherein more than 50% by weight of the filler particleshave a maximum dimension of between 25 and 75 micrometers.
 12. Aninsulator according to any one of the preceding claims, wherein thefiller comprises between 10% and 40%, and preferably between 30% and33%, of the volume of the stress-controlling material layer.
 13. Aninsulator according to any one of the preceding claims, wherein thematrix of the stress-controlling layer comprises a polymeric material, aresin, a thixotropic paint, or a gel.
 14. An insulator according toclaim 13, wherein the polymeric material comprises polyethylene,silicone, or EPDM.
 15. An insulator according to any one of thepreceding claims, wherein the layer of stress-controlling material isapplied directly onto the layer of insulating material, preferably byextrusion, moulding or recovery.
 16. A high voltage bushing, switch, ordisconnector, comprising an insulator according to any one of thepreceding claims.
 17. A high voltage electric cable having astress-controlled termination at one end thereof enclosed within aninsulator according to any one of claims 1 to
 15. 18. Electrical stresscontrolling material comprising a slurry, glaze or paint, into which aredispersed particles capable of providing a stress gradingcharacteristic.
 19. Electrical stress-controlling material according toclaim 18, wherein the slurry, glaze or paint has been fired so as toproduce a material having an electrical stress-controlling switchingcharacteristic.
 20. Electrical stress controlling material according toclaim 18 or 19, wherein the particles are not fired before beingintroduced into the slurry, glaze or paint.
 21. Electrical stresscontrolling material according to any one of claims 18 or 20, whereinthe particulate material comprises zinc oxide filler particles asdefined in claim
 6. 22. Electrical stress controlling material accordingto any one of claims 18 to 21, wherein the slurry forms a ceramicmaterial, preferably porcelain.
 23. Electrical stress controllingmaterial according to any one of claims 18 to 21, wherein the slurrycomprises an inorganic matrix.
 24. An electrical insulator or otherelectrical article or equipment, to which has been applied electricalstress controlling material according to any of claim 18 to
 23. 25. Anelectrical insulator, shed, or other electrical article or equipmenthaving a casing (excluding layers of slurry, glaze, or paint) ofpolymeric or other composition filled with zinc oxide particles asdefined in claim 6.